Short-range differential pulse voltammetry for fast, selective analysis of basal levels of cerebral compounds in vivo.

Differential pulse voltammetry (DPV) with pretreated biosensors (carbon fibre microelectrodes (mCFE), 10-30 microns diameter) allows selective in vivo measurement of basal endogenous levels of dopamine (DA), serotonin (5-HT), their metabolites (dihydroxyphenylacetic acid, DOPAC; 5-hydroxyindoleacetic acid, 5-HIAA), and neuropeptides. We have now modified DPV in order to reduce the time of analysis from tens of seconds to 1-2 s without losing selectivity. We call this newly reported method short-range differential pulse voltammetry (SRDPV). Simply, while in DPV the complete oxidation peak is recorded, SRDPV measures only the top of each oxidation peak. For example, to monitor peak 2 which corresponds to the in vivo oxidation of extracellular DOPAC and occurs at approximately +85 +/- 10 mV, the initial (Ei) and final (Ef) potentials applied with DPV were -100 mV and +200 mV, respectively, while they were +75 mV (Ei) and +95 mV (Ef) with SRDPV. At the typical scan range of 10 mV.s-1, the effective time of measurement was 30 s for DPV and 2 s for SRDPV. A similar procedure was performed to analyze peak 3 (5-HIAA, occurring at +230 +/- 11 mV) with Ei + 50 mV and Ef + 350 mV for DPV, or +220 mV and +240 mV for SRDPV. DPV and SRDPV were compared in vitro by quantitating DOPAC and 5-HIAA in solutions of increasing concentrations (chosen on the basis of the suggested in vivo content of these two compounds). Data indicated that similar sensitivity and selectivity were obtained with both methods at all concentrations, supporting the applicability of SRDPV for in vitro studies. In vivo experiments were performed in anesthetized adult male rats prepared for voltammetry by inserting the electrically pretreated biosensor (mCFE) into the striatum. DPV measurements were performed automatically every 3-5 min and were alternated every 10-20 min with a sequence of 5-10 SRDPV scans performed every 10-30 s. Subsequent pharmacological or electrical manipulations of the two biogenic amine systems studied were monitored by alternate use of DPV and SRDPV. The data presented support the capability of SRDPV with pretreated biosensors to measure in vivo electroactive compounds with selectivity and sensitivity comparable to that of DPV, but with improved time resolution.

Can voltammetry measure nitrogen monoxide (NO) and/or nitrites?

Recently, voltammetry with carbon fibre electrodes (CFE) has been implemented for real time measurement of nitrogen monoxide (NO) indicating that it is oxidised at the potential value of nitrites, approximately +700 mV. In contrast, here we show that modified CFE can monitor NO at oxidation potentials different than that of nitrites, i.e. +550 mV. Indeed, at +550 mV a significant increase of amperometric current levels was obtained when NO but not nitrites, were added to a phosphate buffer saline solution (PBS). Differential pulse voltammetry (DPV) supports these findings as two oxidation peaks were obtained when examining air preserved NO; peak 1 at +550 mV and peak 2 at +700 mV, respectively. In contrast, only peak 2 was monitored when nitrites or a solution of NO oxidised in air was added to PBS. Biological support to these in vitro data comes from the observation that the relaxation of an adrenaline-contracted aortic ring produced via addition of NO is concomitant with peak 1 at +550 mV. The relaxation is almost completed before the appearance of peak 2 at +700 mV. Furthermore, in vivo experiments performed in the striatum of rats show that the amperometric signal monitored at +550 mV is responsive to glutamatergic stimulation or inhibition of NO synthase.

In situ EPR and UV-vis spectroelectrochemistry of hole-transporting organic substrates.

A newly developed in situ electron paramagnetic resonance (EPR)/ultraviolet-visible (UV-vis) spectroelectrochemical cell equipped with a laminated indium-tin oxide (ITO) working electrode was used in the investigation of various organic substrates which are potential hole-transporting materials. The experiment demonstrated the possibility of using such a technique for examining redox behavior of conducting polymers (polypyrrole, PPy), oligomers (thiophene dimmer and quarterthiophene) and bis-anilines (N,N,N',N'-tetraphenylbenzidine, TPB). All investigated structures formed stable paramagnetic intermediates in the first oxidation step characterised with UV-vis spectra in the region 400-600 nm. In the second oxidation step EPR-silent di-cationic structures are formed with broad vis bands in the region 600-1000 nm. The measurement of the reference UV-vis spectra direct in the EPR cavity was possible using a specially-constructed non-contacted ITO plate in the spectroelectrochemical cell in the case of polypyrrole.

LIGA-electrodes in voltammetric and spectroelectrochemical studies.

The advantages of lithographic-galvanic (LIGA) fabricated microstructured honeycomb electrodes are demonstrated for spectroelectrochemical cells with respect to the response time (the time necessary to generate the product in a sufficient layer thickness close to the electrode to be detectable by UV-Vis-NIR spectroscopy) and to the conversion of the redox system in solution under thin-layer conditions. Transmission UV-Vis-NIR spectroscopy for several electrochemical applications can be performed in a special spectroelectrochemical cell based on the LIGA electrode and the two quartz rods, forming the walls of the cell and conducting the light beam through the cell. They are limiting the diffusion layer at the structured part of the working LIGA electrode. These microstructured LIGA-electrodes can be used as well defined models of porous electrodes at which redox processes occur under finite diffusion conditions. Such electrodes have been successfully used in the voltammetric and spectroelectrochemical study of various redox systems in both aqueous and non-aqueous solvents. The possibility to fabricate the well defined microstructures from various organic conducting polymers is demonstrated by the electrochemical deposition of polypyrrole in moulded LIGA-forms at high current densities in aqueous solutions.